This presentation will discuss how the use and need for voltage transformers has changed over the last twenty years. With the introduction of auto-ranging electric meters, meter technicians need to be prepared, use the appropriate tools and PPE for high capacity circuits (without VT's), as this method has become increasingly popular.

1. 10/02/2012 Slide 1
Fusing Metering
Voltages for High
Capacity Circuits
Notes from the Field
Prepared by Tom Lawton,TESCO
for the Spring ECNE Engineering & Operations Conference 2015

2. • Fused test switches have been around for many years.
• Fused potentials have been around for many years.
• However over the past twenty years they have boomed in popularity and
boomed in usage.
• In the same time period fused test leads on field meter testing equipment
have gone from a once in a blue moon request to a standard request.
What is the Issue?

3. • With the advent of auto-ranging meters Utilities
started to recognize that VT’s were no longer
necessary on every service.
• And this is true. But…….
What has changed?

4. • The voltage transformer is designed to connect in parallel with the line to transform the line voltage to 115 or 120
volts suitable for the meter or relay. To keep the voltage at the meters and relays at a safe value, the secondary
circuit must be grounded. Source ABB
• The name instrument transformer is a general classification applied to current and voltage devices used to
change currents and voltages from one magnitude to another or to perform an isolating function, that is, to isolate
the utilization current or voltage from the supply voltage for safety to both the operator and the end device in use.
Instrument transformers are designed specifically for use with electrical equipment falling into the broad category
of devices commonly called instruments such as voltmeters, ammeters, wattmeters, watt-hour meters, protection
relays, etc. Source GE
• VT’s not only step down the voltage to 120v for metering purposes, but they provide a safety feature to the circuit.
The VT is a current limiting component in the circuit.
• The fault current for a VT is typically the same or less than the fault amps of the meter (typ. 10,000 amps).
Voltage Transformers as Safety Devices?

5. • Within recent years, auto-ranging meters have been
introduced.
• VT’s have been eliminated for many applications.
• This means the field technician is now dealing with the
primary voltage.
Autoranging meters

6. A little about Fault Currents

7. • A fuse or breaker designed to
interrupt the largest potential
short-circuit currents can be
helpful on some days.
Short Circuit Current > Interrupt Rating =

8. • Overcurrent protective devices must be selected for the proper
– Voltage
– Current
– Interrupt rating
• This last one is the one most typically overlooked. This is also the one most
people do not understand how to plan for or calculate.
• Current limiting is a type of over current protection
• An overcurrent protective device must be able to withstand the destructive
energy of short-circuit currents.
A little about Fault Currents

9. Short-Circuit Currents & Interrupting Rating

10. Short-Circuit Currents & Interrupting Rating

11. • SCCR’s represent the maximum level of short-circuit current that the component or
equipment can withstand under fault conditions .
• This is used for determining compliance with NEC® 110.10.
• This rating can be marked on individual components or assemblies.
• Assembly ratings take into account all components contained within the equipment
rather than just the main overcurrent protective device.
• A common mistake is to assume that the interrupting rating of the overcurrent
protective device protecting the circuit represents the SCCR for the entire circuit.
Interrupting ratings, used for compliance with NEC® 110.9, apply solely to the
overcurrent protective device. It is the characteristics of the overcurrent protective
device (e.g. opening time, let-through energy) that need to be used in determining
compliance with NEC® 110.10, not the interrupting rating.
• NOTE: Short-circuit current ratings (SCCRs) are different than interrupting ratings
marked on overcurrent protective devices.
What is a Short-Circuit Current Rating
(SCCR) ?

12. • For meter disconnect switches, this withstand
level (SCCR), is often determined by product
testing. For assemblies, the marking can be
determined through product listing or by an
approved method.
• Any valid method uses the “weakest link”
approach. The marking should represent the
limits of the assembly for a safe installation.
How is SCCR Determined?

13. Point-to-Point Method of Short-Circuit
Calculations

14. • Some have added fused potentials in line or at the test switch
for transformer rated services.
What have utilities changed as a result of losing
the isolation and current limiting features of a
voltage transformer?

15. • Some have gone to fused test leads for their field technicians.
Fusing The Test Switch

16. Meter Test Switch Configurations
• Layout
• Handle Colors
• Reversed vs. Normal Potentials
• Current Links
• Base Sizing
• Barrier Locations
NOTE: On installations that contain Test Switches, test leads terminated
with a test switch safety test probe (test plug) should be used for CT
testing. This provides a “make-before-break” connection to prevent
accidental opening of the current transformer secondary loop.

17. Fitting into a Pre-Wired Transformer Rated
Enclosure?
• Move items around
• Small nipple box with
four pole switch base
and fusing

18. • Some have gone to cold sequencing for self contained
services.
Cold Sequencing

19. • Some rely on better PPE for their Meter Techs
PPE

20. • Some have not changed their metering practices at all.
• Utilities are not required to fuse the circuits.
• Do we really need to do anything?
None of the Above!

21. Questions and Discussion
Tom Lawton
TESCO – The Eastern Specialty Company
Bristol, PA
Tom.Lawton@tescometermanager.com
Cell: 215-688-0298
This presentation can also be found under Meter Conferences
and Schools on the TESCO web site: www.tesco-advent.com